bpo-42747: Remove Py_TPFLAGS_HAVE_AM_SEND and make Py_TPFLAGS_HAVE_VERSION_TAG no-op (GH-27260)

* Remove code that checks Py_TPFLAGS_HAVE_VERSION_TAG
    
    The field is always present in the type struct, as explained
    in the added comment.

* Remove Py_TPFLAGS_HAVE_AM_SEND
    
    The flag is not needed, and since it was added in 3.10 it can be removed now.

7 files changed, 24 insertions, 56 deletions

diff --git a/Doc/c-api/typeobj.rst b/Doc/c-api/typeobj.rst
index 004cecd..b18e385 100644
--- a/Doc/c-api/typeobj.rst
+++ b/Doc/c-api/typeobj.rst
@@ -1098,8 +1098,7 @@ and :c:type:`PyType_Type` effectively act as defaults.)
 
       This is a bitmask of all the bits that pertain to the existence of certain
       fields in the type object and its extension structures. Currently, it includes
-      the following bits: :const:`Py_TPFLAGS_HAVE_STACKLESS_EXTENSION`,
-      :const:`Py_TPFLAGS_HAVE_VERSION_TAG`.
+      the following bits: :const:`Py_TPFLAGS_HAVE_STACKLESS_EXTENSION`.
 
       **Inheritance:**
 
@@ -1179,14 +1178,6 @@ and :c:type:`PyType_Type` effectively act as defaults.)
 
       .. versionadded:: 3.9
 
-
-   .. data:: Py_TPFLAGS_HAVE_AM_SEND
-
-      This bit is set when the :c:member:`~PyAsyncMethods.am_send` entry is present in the
-      :c:member:`~PyTypeObject.tp_as_async` slot of type structure.
-
-      .. versionadded:: 3.10
-
    .. data:: Py_TPFLAGS_IMMUTABLETYPE
 
       This bit is set for type objects that are immutable: type attributes cannot be set nor deleted.
diff --git a/Include/object.h b/Include/object.h
index 109f535..9e6a8f4 100644
--- a/Include/object.h
+++ b/Include/object.h
@@ -368,18 +368,12 @@ given type object has a specified feature.
 /* Objects behave like an unbound method */
 #define Py_TPFLAGS_METHOD_DESCRIPTOR (1UL << 17)
 
-/* Objects support type attribute cache */
-#define Py_TPFLAGS_HAVE_VERSION_TAG   (1UL << 18)
+/* Object has up-to-date type attribute cache */
 #define Py_TPFLAGS_VALID_VERSION_TAG  (1UL << 19)
 
 /* Type is abstract and cannot be instantiated */
 #define Py_TPFLAGS_IS_ABSTRACT (1UL << 20)
 
-#if !defined(Py_LIMITED_API) || Py_LIMITED_API+0 >= 0x030A0000
-/* Type has am_send entry in tp_as_async slot */
-#define Py_TPFLAGS_HAVE_AM_SEND (1UL << 21)
-#endif
-
 // This undocumented flag gives certain built-ins their unique pattern-matching
 // behavior, which allows a single positional subpattern to match against the
 // subject itself (rather than a mapped attribute on it):
@@ -397,19 +391,23 @@ given type object has a specified feature.
 
 #define Py_TPFLAGS_DEFAULT  ( \
                  Py_TPFLAGS_HAVE_STACKLESS_EXTENSION | \
-                 Py_TPFLAGS_HAVE_VERSION_TAG | \
                 0)
 
-/* NOTE: The following flags reuse lower bits (removed as part of the
+/* NOTE: Some of the following flags reuse lower bits (removed as part of the
  * Python 3.0 transition). */
 
-/* The following flag is kept for compatibility. Starting with 3.8,
- * binary compatibility of C extensions across feature releases of
- * Python is not supported anymore, except when using the stable ABI.
+/* The following flags are kept for compatibility; in previous
+ * versions they indicated presence of newer tp_* fields on the
+ * type struct.
+ * Starting with 3.8, binary compatibility of C extensions across
+ * feature releases of Python is not supported anymore (except when
+ * using the stable ABI, in which all classes are created dynamically,
+ * using the interpreter's memory layout.)
+ * Note that older extensions using the stable ABI set these flags,
+ * so the bits must not be repurposed.
  */
-
-/* Type structure has tp_finalize member (3.4) */
 #define Py_TPFLAGS_HAVE_FINALIZE (1UL << 0)
+#define Py_TPFLAGS_HAVE_VERSION_TAG   (1UL << 18)
 
 
 /*
diff --git a/Misc/NEWS.d/next/C API/2021-07-20-16-21-06.bpo-42747.rRxjUY.rst b/Misc/NEWS.d/next/C API/2021-07-20-16-21-06.bpo-42747.rRxjUY.rst
new file mode 100644
index 0000000..c7ac5a7
--- /dev/null
+++ b/Misc/NEWS.d/next/C API/2021-07-20-16-21-06.bpo-42747.rRxjUY.rst
@@ -0,0 +1,4 @@
+The ``Py_TPFLAGS_HAVE_VERSION_TAG`` type flag now does nothing. The
+``Py_TPFLAGS_HAVE_AM_SEND`` flag (which was added in 3.10) is removed. Both
+were unnecessary because it is not possible to have type objects with the
+relevant fields missing.
diff --git a/Modules/_asynciomodule.c b/Modules/_asynciomodule.c
index a4d5d45..ecc73d1c 100644
--- a/Modules/_asynciomodule.c
+++ b/Modules/_asynciomodule.c
@@ -1764,8 +1764,7 @@ static PyTypeObject FutureIterType = {
     .tp_dealloc = (destructor)FutureIter_dealloc,
     .tp_as_async = &FutureIterType_as_async,
     .tp_getattro = PyObject_GenericGetAttr,
-    .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC |
-        Py_TPFLAGS_HAVE_AM_SEND,
+    .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,
     .tp_traverse = (traverseproc)FutureIter_traverse,
     .tp_iter = PyObject_SelfIter,
     .tp_iternext = (iternextfunc)FutureIter_iternext,
diff --git a/Objects/abstract.c b/Objects/abstract.c
index f14a923..842c367 100644
--- a/Objects/abstract.c
+++ b/Objects/abstract.c
@@ -2804,9 +2804,7 @@ PyIter_Send(PyObject *iter, PyObject *arg, PyObject **result)
     _Py_IDENTIFIER(send);
     assert(arg != NULL);
     assert(result != NULL);
-    if (PyType_HasFeature(Py_TYPE(iter), Py_TPFLAGS_HAVE_AM_SEND)) {
-        assert (Py_TYPE(iter)->tp_as_async != NULL);
-        assert (Py_TYPE(iter)->tp_as_async->am_send != NULL);
+    if (Py_TYPE(iter)->tp_as_async && Py_TYPE(iter)->tp_as_async->am_send) {
         PySendResult res = Py_TYPE(iter)->tp_as_async->am_send(iter, arg, result);
         assert(_Py_CheckSlotResult(iter, "am_send", res != PYGEN_ERROR));
         return res;
diff --git a/Objects/genobject.c b/Objects/genobject.c
index 27fe15c..8cc965a 100644
--- a/Objects/genobject.c
+++ b/Objects/genobject.c
@@ -782,8 +782,7 @@ PyTypeObject PyGen_Type = {
     PyObject_GenericGetAttr,                    /* tp_getattro */
     0,                                          /* tp_setattro */
     0,                                          /* tp_as_buffer */
-    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC |
-        Py_TPFLAGS_HAVE_AM_SEND,                /* tp_flags */
+    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,    /* tp_flags */
     0,                                          /* tp_doc */
     (traverseproc)gen_traverse,                 /* tp_traverse */
     0,                                          /* tp_clear */
@@ -1029,8 +1028,7 @@ PyTypeObject PyCoro_Type = {
     PyObject_GenericGetAttr,                    /* tp_getattro */
     0,                                          /* tp_setattro */
     0,                                          /* tp_as_buffer */
-    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC |
-        Py_TPFLAGS_HAVE_AM_SEND,                /* tp_flags */
+    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,    /* tp_flags */
     0,                                          /* tp_doc */
     (traverseproc)gen_traverse,                 /* tp_traverse */
     0,                                          /* tp_clear */
@@ -1415,8 +1413,7 @@ PyTypeObject PyAsyncGen_Type = {
     PyObject_GenericGetAttr,                    /* tp_getattro */
     0,                                          /* tp_setattro */
     0,                                          /* tp_as_buffer */
-    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC |
-        Py_TPFLAGS_HAVE_AM_SEND,                /* tp_flags */
+    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,    /* tp_flags */
     0,                                          /* tp_doc */
     (traverseproc)async_gen_traverse,           /* tp_traverse */
     0,                                          /* tp_clear */
diff --git a/Objects/typeobject.c b/Objects/typeobject.c
index badd706..42a9978 100644
--- a/Objects/typeobject.c
+++ b/Objects/typeobject.c
@@ -324,10 +324,6 @@ PyType_Modified(PyTypeObject *type)
 
        Invariants:
 
-       - Py_TPFLAGS_VALID_VERSION_TAG is never set if
-         Py_TPFLAGS_HAVE_VERSION_TAG is not set (in case of a
-         bizarre MRO, see type_mro_modified()).
-
        - before Py_TPFLAGS_VALID_VERSION_TAG can be set on a type,
          it must first be set on all super types.
 
@@ -379,9 +375,6 @@ type_mro_modified(PyTypeObject *type, PyObject *bases) {
     PyObject *mro_meth = NULL;
     PyObject *type_mro_meth = NULL;
 
-    if (!_PyType_HasFeature(type, Py_TPFLAGS_HAVE_VERSION_TAG))
-        return;
-
     if (custom) {
         mro_meth = lookup_maybe_method(
             (PyObject *)type, &PyId_mro, &unbound);
@@ -404,8 +397,7 @@ type_mro_modified(PyTypeObject *type, PyObject *bases) {
         assert(PyType_Check(b));
         cls = (PyTypeObject *)b;
 
-        if (!_PyType_HasFeature(cls, Py_TPFLAGS_HAVE_VERSION_TAG) ||
-            !PyType_IsSubtype(type, cls)) {
+        if (!PyType_IsSubtype(type, cls)) {
             goto clear;
         }
     }
@@ -413,8 +405,7 @@ type_mro_modified(PyTypeObject *type, PyObject *bases) {
  clear:
     Py_XDECREF(mro_meth);
     Py_XDECREF(type_mro_meth);
-    type->tp_flags &= ~(Py_TPFLAGS_HAVE_VERSION_TAG|
-                        Py_TPFLAGS_VALID_VERSION_TAG);
+    type->tp_flags &= ~Py_TPFLAGS_VALID_VERSION_TAG;
     type->tp_version_tag = 0; /* 0 is not a valid version tag */
 }
 
@@ -431,8 +422,6 @@ assign_version_tag(struct type_cache *cache, PyTypeObject *type)
 
     if (_PyType_HasFeature(type, Py_TPFLAGS_VALID_VERSION_TAG))
         return 1;
-    if (!_PyType_HasFeature(type, Py_TPFLAGS_HAVE_VERSION_TAG))
-        return 0;
     if (!_PyType_HasFeature(type, Py_TPFLAGS_READY))
         return 0;
 
@@ -5978,14 +5967,6 @@ type_ready_pre_checks(PyTypeObject *type)
         _PyObject_ASSERT((PyObject *)type, type->tp_call != NULL);
     }
 
-    /* Consistency check for Py_TPFLAGS_HAVE_AM_SEND - flag requires
-     * type->tp_as_async->am_send to be present.
-     */
-    if (type->tp_flags & Py_TPFLAGS_HAVE_AM_SEND) {
-        _PyObject_ASSERT((PyObject *)type, type->tp_as_async != NULL);
-        _PyObject_ASSERT((PyObject *)type, type->tp_as_async->am_send != NULL);
-    }
-
     /* Consistency checks for pattern matching
      * Py_TPFLAGS_SEQUENCE and Py_TPFLAGS_MAPPING are mutually exclusive */
     _PyObject_ASSERT((PyObject *)type, (type->tp_flags & COLLECTION_FLAGS) != COLLECTION_FLAGS);